1. Field of the Invention
This invention relates to broadband antenna design and, more particularly, to a log-periodic dipole array (LPDA) antenna with improved performance over a broad frequency range.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Log-periodic dipole array (LPDA) antennas are popular broadband antennas for many applications. In general, an LPDA antenna includes a collection of linear or tapered dipoles, which are scaled and arranged in a log-periodic array. Each dipole within the array comprises two elements or halves, which vary in length and extend outward from a pair of transmission line structures (i.e., “feed conductors”). The dipoles are arranged from shortest to longest, such that the length and spacing between dipole elements varies logarithmically along the antenna. In addition, the dipole lengths and spacings are related to the frequency range over which the antenna is configured to operate. For example, the length of the longest dipole is proportional to the lowest operating frequency, while the length of the shortest dipole is proportional to the highest operating frequency of the LPDA antenna. In order to provide a relatively broad frequency range, a relatively large LPDA antenna having a great discrepancy between the lengths of the longest and shortest elements is typically needed.
In many cases, the dipoles are constructed from aluminum bar stock having a cylindrical cross-section. However, other conductive materials (such as copper and its alloys) and cross-sections (such as rectangular) may also be used to fabricate the dipole elements. In most cases, the dipole elements are attached to the feed conductors using screws or other mechanical fasteners. As an alternative, the dipole elements may be individually soldered or welded to the feed conductors. However, soldering and welding are seldom used, because the intense localized heating required by these processes tends to distort the antenna structure.
During use, the LPDA is oriented such that the end with the shortest elements (i.e., the front end) is pointed in the desired direction of transmission or reception. In most cases, the antenna is fed at the front end to avoid pattern distortions. For example, the feed conductors are usually spaced apart and arranged in a plane perpendicular to the dipole elements. In some cases, the antenna may be fed by running a coaxial feed line along the interior of one of the feed conductors to which the dipole elements are connected. Such a configuration is typically referred to as an “over/under feed mechanism.”
Bringing the feed signal to the front of the antenna serves two purposes. First, it allows the connector to the signal source or receiver to be realized at the back end of the antenna (i.e., the end with the longest elements), which provides a significant mechanical advantage. Second, feeding the antenna at the front reduces pattern distortions and provides an intrinsic balancing network. For example, the coaxial feed line may be fully contained inside one of the two feed conductors of the over/under feed mechanism. At the front of the antenna (i.e., the “feed region”), the inner conductor of the coaxial feed line may protrude from one conductor and connect to the other conductor. If the feed region is electrically small, current continuity will be maintained and the currents flowing along the two conductors will be balanced.
The above feed arrangement is often referred to as an “infinite balun.” Although not technically a balun, the feed arrangement provides an intrinsic current balance for the antenna, thereby eliminating the need for an additional balancing transformer. By feeding the antenna at the front end (i.e., at the smaller, high frequency elements), no blockage occurs and the antenna provides a unidirectional pattern that is maintained over a broad frequency range.
In order to direct the antenna's radiation “forward” even though it is being fed “backwards,” successive dipole elements must be fed by signals 180° out of phase. This is achieved by electrically connecting each feed conductor to alternating halves of the successive dipoles. For example, a feed conductor may be electrically connected to the “left” element of one dipole pair, followed by the “right” element of the next dipole pair, and so on.
The most successful LPDA designs available today combine the “infinite balan” technique with the over/under feed mechanism discussed above. However, traditional LPDA designs incorporating these techniques still present many disadvantages. For example, conventional LPDA antennas that use screws (or other mechanical fasteners) to attach the dipole elements to the feed conductors often suffer from intermittent electrical contact at the base of the elements (i.e., at the connection points between the dipole elements and the feed conductors). In other words, thermal expansion of the dipole elements cause the fasteners to loosen over time, allowing moisture and oxygen in between the base of the elements and the feed conductors. This leads to unavoidable oxidation and intermittent electrical contact at the base of the elements. In some cases, the electrical contact problem may be solved by soldering or welding the dipole elements directly to the feed conductors, as noted above. However, soldering and welding require intense localized heating, which tends to distort the antenna structure. For this reason, mechanical fasteners (such as screws) are almost primarily used to attach the dipole elements to the feed conductors.
In addition, LPDA designs employing dipole elements attached with mechanical fasteners become impractical at high operating frequencies (e.g., at about microwave frequencies and above). As noted above, the lengths of the dipole elements become increasingly shorter as the high frequency limit of the operating frequency range increases. In most cases, the cost associated with each dipole element is similar, regardless of element size, because the same machining processes are involved in the manufacture of each element. Thus, it becomes very expensive to extend the high frequency limit of a traditional LPDA antenna into the microwave frequency range. In addition, the over/under feed mechanism necessarily staggers the two halves of each dipole to accommodate higher frequency limits. However, staggering introduces cross-polarized radiated fields, which can only be minimized by reducing the size of the feed geometry. This often results in power handling problems and increases the difficulty of assembly.
One approach to fabricating an LPDA antenna with an increased high frequency limit is to implement the antenna on a printed circuit board (PCB). For example, U.S. Pat. No. 5,903,670 to Braathen provides an LPDA antenna in which the dipole elements and one feed conductor are patterned onto one side of an insulating substrate, while a second feed conductor is patterned onto an opposite side of the substrate. The feed conductors are implemented as micro-strip lines, which may be embedded within the substrate or coupled to top and bottom surfaces of the substrate. Phase transposition is provided by connecting the second feed conductor to alternating dipole elements through vias formed within the substrate. In this manner, the dielectric substrate supports the dipole elements and keeps them in the desired co-planar configuration, while the vias connect the second feed conductor to the dipole elements at various points.
Even though LPDA antennas built using printed circuit technology enable high frequency operation, they provide their own set of disadvantages. For example, the dielectric substrate of any printed circuit necessarily perturbs the electromagnetic field generated by the antenna, even if it is of low permittivity. Perhaps the best available substrates (e.g., PTFE based substrates) exhibit a relative permittivity of about 2.0. Even these substrates cause a significant perturbation of the electromagnetic field, which ultimately degrades the intended radiation pattern.
In addition, printed circuit antennas are typically limited to operating over a narrow, high frequency range and not readily or inexpensively adapted for operating over relatively larger frequency ranges. Attempts have been made to combine smaller, printed circuit LPDA antennas with larger, traditionally-fabricated LPDA antennas to cover relatively large frequency ranges. However, the marriage of two dissimilar LPDAs (i.e., the presence of dielectric in the printed circuit based LPDA and the absence of dielectric in the traditional LPDA necessarily makes them dissimilar) inevitably results in some performance degradation, especially in the cross-over region (i.e., the region arranged about the upper frequency limit of the traditional LPDA and the lower frequency limit of the printed circuit LPDA). The presence of a dielectric substrate also tends to degrade the frequency independent nature of the LPDA antenna.
Therefore, a need remains for an improved LPDA antenna design. In particular, the improved LPDA design would overcome the above-mentioned problems associated with both traditional and printed circuit LPDA designs.